Add support for graceful shutdown of OPA server

There is a new CLI option to configure the grace period:

`--shutdown-grace-period`

The option defaults to 10 seconds.

When a SIGINT or SIGTERM is sent the runtime will catch them and
attempt to gracefully shutdown the http servers. If the timeout is reached
it will log an error and continue with exiting (in a less graceful way).

Unit testing is a little bit light on this. Its touching some parts of the
code that aren't super easy to mock out. We can probably refactor
things in the future to make it easier.

Fixes: #1291
Signed-off-by: Patrick East <east.patrick@gmail.com>